Silicon carbide semiconductor device and method of manufacturing silicon carbide semiconductor device

ABSTRACT

A silicon carbide substrate includes a first impurity region having a first conductivity type, a second impurity region having a second conductivity type, a third impurity region having the first conductivity type, and a fourth impurity region provided between a second main surface and a bottom surface and having the second conductivity type. The first impurity region has a first region being in contact with the second impurity region and having a first impurity concentration, a second region being continuous to the first region, provided between the first region and the second main surface, and having a second impurity concentration lower than the first impurity concentration, and a third region being continuous to the first region and having a third impurity concentration higher than the first impurity concentration. A side surface is in contact with the third region, the second impurity region, and the third impurity region.

TECHNICAL FIELD

The present disclosure relates to a silicon carbide semiconductor deviceand a method of manufacturing a silicon carbide semiconductor device.The present application claims priority to Japanese Patent ApplicationNo. 2018-150518 filed on Aug. 9, 2018, the entire contents of which areincorporated herein by reference.

BACKGROUND ART

Japanese Patent Laying-Open No. 2015-072999 (PTL 1) describes a metaloxide semiconductor field effect transistor (MOSFET) having a trenchgate structure. This MOSFET has a p type bottom layer formed to coverthe bottom of a trench, and an n type current dispersion layer formedbetween a p type base region and an n type drift layer.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Laying-Open No. 2015-072999

SUMMARY OF INVENTION

A silicon carbide semiconductor device according to the presentdisclosure includes a silicon carbide substrate. The silicon carbidesubstrate has a first main surface and a second main surface opposite tothe first main surface. The first main surface is provided with a trenchhaving a side surface and a bottom surface continuous to the sidesurface. The silicon carbide substrate includes a first impurity regionlocated between the first main surface and the second main surface andhaving a first conductivity type, a second impurity region locatedbetween the first main surface and the first impurity region, providedon the first impurity region, and having a second conductivity typedifferent from the first conductivity type, a third impurity regionlocated between the first main surface and the second impurity region,provided on the second impurity region so as to be separated from thefirst impurity region, and having the first conductivity type, and afourth impurity region provided between the second main surface and thebottom surface and having the second conductivity type. The firstimpurity region has a first region being in contact with the secondimpurity region and having a first impurity concentration, a secondregion being continuous to the first region, provided between the firstregion and the second main surface, and having a second impurityconcentration lower than the first impurity concentration, and a thirdregion being continuous to the first region and having a third impurityconcentration higher than the first impurity concentration. The sidesurface is in contact with the third region, the second impurity region,and the third impurity region.

A method of manufacturing a silicon carbide semiconductor deviceaccording to the present disclosure includes the following steps. Asilicon carbide substrate having a first main surface and a second mainsurface opposite to the first main surface is prepared. The siliconcarbide substrate includes a first impurity region located between thefirst main surface and the second main surface and having a firstconductivity type, a second impurity region located between the firstmain surface and the first impurity region, provided on the firstimpurity region, and having a second conductivity type different fromthe first conductivity type, and a third impurity region located betweenthe first main surface and the second impurity region, provided on thesecond impurity region so as to be separated from the first impurityregion, being in contact with the first main surface, and having thefirst conductivity type. The first impurity region has a first region incontact with the second impurity region, and a second region beingcontinuous to the first region and located opposite to the secondimpurity region with respect to the first region. A mask is furtherformed on the first main surface. The third impurity region and thesecond impurity region are etched using the mask, to form a first trenchin the first main surface. Ions are implanted into the first trenchusing the mask, to form a fourth impurity region being in contact withthe first impurity region and having the second conductivity type. Ionsare implanted into the first trench using the mask, to form a thirdregion being in contact with the first impurity region and having thefirst conductivity type. The third region, the second impurity regionand the fourth impurity region are etched using the mask, to extend thefirst trench to form a second trench. A side surface of the secondtrench is in contact with the third region, the second impurity region,and the third impurity region.

A method of manufacturing a silicon carbide semiconductor deviceaccording to the present disclosure includes the following steps. Asilicon carbide substrate having a first main surface and a second mainsurface opposite to the first main surface is prepared. The siliconcarbide substrate includes a first impurity region located between thefirst main surface and the second main surface and having a firstconductivity type, a second impurity region located between the firstmain surface and the first impurity region, provided on the firstimpurity region, and having a second conductivity type different fromthe first conductivity type, and a third impurity region located betweenthe first main surface and the second impurity region, provided on thesecond impurity region so as to be separated from the first impurityregion, being in contact with the first main surface, and having thefirst conductivity type. The first impurity region has a first region incontact with the second impurity region, and a second region beingcontinuous to the first region and located opposite to the secondimpurity region with respect to the first region. A mask is furtherformed on the first main surface. The third impurity region and thesecond impurity region are etched using the mask, to form a first trenchin the first main surface. Ions are implanted into the first trenchusing the mask, to form a third region being in contact with the firstimpurity region and having the first conductivity type. The third regionand the second impurity region are etched using the mask, to extend thefirst trench to form a second trench. Ions are implanted into the secondtrench using the mask, to form a fourth impurity region being in contactwith the first impurity region and having the second conductivity type.A side surface of the second trench is in contact with the third region,the second impurity region, and the third impurity region.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing the structure of asilicon carbide semiconductor device according to a first embodiment.

FIG. 2 is a schematic plan view showing the structure of a trench in thesilicon carbide semiconductor device according to the first embodiment.

FIG. 3 is a flow diagram schematically showing a method of manufacturingthe silicon carbide semiconductor device according to the firstembodiment.

FIG. 4 is a schematic cross-sectional view showing a first step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 5 is a schematic cross-sectional view showing a second step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 6 is a schematic cross-sectional view showing a third step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 7 is a schematic cross-sectional view showing a fourth step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 8 is a schematic cross-sectional view showing a fifth step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 9 is a schematic cross-sectional view showing a sixth step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 10 is a schematic cross-sectional view showing a seventh step ofthe method of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 11 is a schematic cross-sectional view showing an eighth step ofthe method of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 12 is a schematic cross-sectional view showing a ninth step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 13 is a schematic cross-sectional view showing a tenth step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 14 is a schematic cross-sectional view showing an eleventh step ofthe method of manufacturing the silicon carbide semiconductor deviceaccording to the first embodiment.

FIG. 15 is a flow diagram schematically showing a method ofmanufacturing a silicon carbide semiconductor device according to avariation of the first embodiment.

FIG. 16 is a schematic cross-sectional view showing a first step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the variation of the first embodiment.

FIG. 17 is a schematic cross-sectional view showing a second step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the variation of the first embodiment.

FIG. 18 is a schematic cross-sectional view showing the structure of asilicon carbide semiconductor device according to a second embodiment.

FIG. 19 shows simulation data illustrating relation between acharacteristic on resistance and a distance from a side surface of atrench, and relation between a breakdown voltage and the distance fromthe side surface of the trench.

DETAILED DESCRIPTION Problem to be Solved by the Present Disclosure

An object of the present disclosure is to provide a silicon carbidesemiconductor device capable of reducing a characteristic on resistancewhile maintaining a high breakdown voltage, and a method ofmanufacturing a silicon carbide semiconductor device.

Advantageous Effect of the Present Disclosure

According to the present disclosure, a silicon carbide semiconductordevice capable of reducing a characteristic on resistance whilemaintaining a high breakdown voltage, and a method of manufacturing asilicon carbide semiconductor device can be provided.

Summary of Embodiments of the Present Disclosure

An overview of embodiments of the present disclosure is initiallyprovided. Regarding crystallographic denotation herein, an individualorientation, a group orientation, an individual plane, and a group planeare shown in [ ], < >, ( ) and { }, respectively. A crystallographicallynegative index is normally expressed by a number with a bar “-”thereabove, however, a negative sign herein precedes a number to expressa crystallographically negative index.

(1) A silicon carbide semiconductor device 200 according to the presentdisclosure includes a silicon carbide substrate 100. Silicon carbidesubstrate 100 has a first main surface 1 and a second main surface 2opposite to first main surface 1. First main surface 1 is provided witha trench 5 having a side surface 3 and a bottom surface 4 continuous toside surface 3. Silicon carbide substrate 100 includes a first impurityregion 10 located between first main surface 1 and second main surface 2and having a first conductivity type, a second impurity region 20located between first main surface 1 and first impurity region 10,provided on first impurity region 10, and having a second conductivitytype different from the first conductivity type, a third impurity region30 located between first main surface 1 and second impurity region 20,provided on second impurity region 20 so as to be separated from firstimpurity region 10, and having the first conductivity type, and a fourthimpurity region 40 provided between second main surface 2 and bottomsurface 4 and having the second conductivity type. First impurity region10 has a first region 11 being in contact with second impurity region 20and having a first impurity concentration, a second region 12 beingcontinuous to first region 11, provided between first region 11 andsecond main surface 2, and having a second impurity concentration lowerthan the first impurity concentration, and a third region 13 beingcontinuous to first region 11 and having a third impurity concentrationhigher than the first impurity concentration. Side surface 3 is incontact with third region 13, second impurity region 20, and thirdimpurity region 30.

(2) In silicon carbide semiconductor device 200 according to (1), fourthimpurity region 40 may be in contact with bottom surface 4.

(3) According to silicon carbide semiconductor device 200 according to(2), in a direction parallel to first main surface 1, a distance fromside surface 3 to an end of third region 13 may be shorter than adistance from side surface 3 to an end of fourth impurity region 40.

(4) In silicon carbide semiconductor device 200 according to (1), fourthimpurity region 40 may be spaced apart from bottom surface 4. Thirdregion 13 may be provided between bottom surface 4 and fourth impurityregion 40, and be in contact with each of bottom surface 4 and fourthimpurity region 40.

(5) In silicon carbide semiconductor device 200 according to (4), thirdregion 13 may be in contact with second region 12. In a directionparallel to first main surface 1, a distance from side surface 3 to anend of third region 13 may be shorter than a distance from side surface3 to an end of fourth impurity region 40.

(6) A method of manufacturing a silicon carbide semiconductor device 200according to the present disclosure includes the following steps. Asilicon carbide substrate 100 having a first main surface 1 and a secondmain surface 2 opposite to first main surface 1 is prepared. Siliconcarbide substrate 100 includes a first impurity region 10 locatedbetween first main surface 1 and second main surface 2 and having afirst conductivity type, a second impurity region 20 located betweenfirst main surface 1 and first impurity region 10, provided on firstimpurity region 10, and having a second conductivity type different fromthe first conductivity type, and a third impurity region 30 locatedbetween first main surface 1 and second impurity region 20, provided onsecond impurity region 20 so as to be separated from first impurityregion 10, being in contact with first main surface 1, and having thefirst conductivity type. First impurity region 10 has a first region 11in contact with second impurity region 20, and a second region 12 beingcontinuous to first region 11 and located opposite to second impurityregion 20 with respect to first region 11. A mask 64 is further formedon first main surface 1. Third impurity region 30 and second impurityregion 20 are etched using mask 64, to form a first trench 70 in firstmain surface 1. Ions are implanted into first trench 70 using mask 64,to form a fourth impurity region 40 being in contact with first impurityregion 10 and having the second conductivity type. Ions are implantedinto first trench 70 using mask 64, to form a third region 13 being incontact with first impurity region 10 and having the first conductivitytype. Third region 13, second impurity region 20 and fourth impurityregion 40 are etched using mask 64, to extend first trench 70 to form asecond trench 5. A side surface 3 of second trench 5 is in contact withthird region 13, second impurity region 20, and third impurity region30.

(7) A method of manufacturing a silicon carbide semiconductor device 200according to the present disclosure includes the following steps. Asilicon carbide substrate 100 having a first main surface 1 and a secondmain surface 2 opposite to first main surface 1 is prepared. Siliconcarbide substrate 100 includes a first impurity region 10 locatedbetween first main surface 1 and second main surface 2 and having afirst conductivity type, a second impurity region 20 located betweenfirst main surface 1 and first impurity region 10, provided on firstimpurity region 10, and having a second conductivity type different fromthe first conductivity type, and a third impurity region 30 locatedbetween first main surface 1 and second impurity region 20, provided onsecond impurity region 20 so as to be separated from first impurityregion 10, being in contact with first main surface 1, and having thefirst conductivity type. First impurity region 10 has a first region 11in contact with second impurity region 20, and a second region 12 beingcontinuous to first region 11 and located opposite to second impurityregion 20 with respect to first region 11. A mask 64 is further formedon first main surface 1. Third impurity region 30 and second impurityregion 20 are etched using mask 64, to form a first trench 70 in firstmain surface 1. Ions are implanted into first trench 70 using mask 64,to form a third region 13 being in contact with first impurity region 10and having the first conductivity type. Third region 13 and secondimpurity region 20 are etched using mask 64, to extend first trench 70to form a second trench 5. Ions are implanted into second trench 5 usingmask 64, to form a fourth impurity region 40 being in contact with firstimpurity region 10 and having the second conductivity type. A sidesurface 3 of second trench 5 is in contact with third region 13, secondimpurity region 20, and third impurity region 30.

(8) In the method of manufacturing silicon carbide semiconductor device200 according to (6) or (7), after forming second trench 5, siliconcarbide substrate 100 may be annealed at not less than 1400° C. and notmore than 1900° C. in a silicon carbide atmosphere.

DETAILS OF EMBODIMENTS OF THE PRESENT DISCLOSURE

The details of the embodiments of the present disclosure are hereinafterdescribed. The same or corresponding elements in the description beloware designated by the same symbols and the same description thereof willnot be repeated.

First Embodiment

The configuration of a MOSFET as a silicon carbide semiconductor deviceaccording to a first embodiment is initially described.

As shown in FIG. 1, a silicon carbide semiconductor device 200 accordingto the present embodiment mainly includes a silicon carbide substrate100, a gate electrode 7, a gate insulating film 6, an interlayerinsulating film 23, a source electrode 8, a source wire 21, a drainelectrode 9, and a protective film 22. Silicon carbide substrate 10 hasa first main surface 1, and a second main surface 2 opposite to firstmain surface 1. Silicon carbide substrate 100 includes a silicon carbidesingle-crystal substrate 15, and a silicon carbide epitaxial layer 16provided on silicon carbide single-crystal substrate 15. Silicon carbidesingle-crystal substrate 15 defines second main surface 2 of siliconcarbide substrate 100. Silicon carbide epitaxial layer 16 defines firstmain surface 1 of silicon carbide substrate 100.

Silicon carbide single-crystal substrate 15 is made of hexagonal siliconcarbide having a polytype of 4H, for example. A maximum diameter offirst main surface 1 of silicon carbide substrate 100 is 150 mm, forexample, and preferably not less than 150 mm. First main surface 1 is a{0001} plane or a plane angled off by not more than 8° relative to the{0001} plane, for example. Specifically, first main surface 1 is a(0001) plane or a plane angled off by not more than 8° relative to the(0001) plane, for example. First main surface 1 may be a (000-1) planeor a plane angled off by not more than 8° relative to the (000-1) plane,for example. Silicon carbide single-crystal substrate 15 has a thicknessof 400 μm, for example. Silicon carbide single-crystal substrate 15 hasa resistivity of 0.017 Ωcm, for example.

Silicon carbide epitaxial layer 16 mainly includes a first impurityregion 10, a base region 20 (second impurity region 20), a source region30 (third impurity region 30), a contact region 42, and a fourthimpurity region 40. First impurity region 10 is an n type (firstconductivity type) region including an n type impurity (donor) forproviding n type such as nitrogen. First impurity region 10 is locatedbetween first main surface 1 and second main surface 2.

First impurity region 10 mainly includes a first region 11, a secondregion 12, and a third region 13. First region 11 is a current spreadlayer. First region 11 is in contact with second impurity region 20.First region 11 has a first impurity concentration. A concentration ofthe n type impurity contained in first region 11 (first impurityconcentration) is not less than 3×10¹⁶ cm⁻³ and not more than 3×10¹⁷cm⁻³, for example. First region 11 has a thickness of not less than 0.2μm and not more than 0.5 μm, for example.

Second region 12 is a drift layer. Second region 12 is provided betweenfirst region 11 and second main surface 2. Second region 12 iscontinuous to first region 11. Second region 12 has a second impurityconcentration lower than the first impurity concentration. Aconcentration of the n type impurity contained in second region 12(second impurity concentration) is 8×10¹⁵ cm³, for example. Secondregion 12 may be greater in thickness than first region 11.

Third region 13 is a local current spread region. Third region 13 iscontinuous to first region 11. Third region 13 has a third impurityconcentration higher than the first impurity concentration. Aconcentration of the n type impurity contained in third region 13 (thirdimpurity concentration) is not less than 2×10¹⁶ cm⁻³ and not more than5×10¹⁷ cm⁻³, for example. The concentration of the n type impuritycontained in second region 12 (second impurity concentration) is lowerthan a concentration of an n type impurity contained in the siliconcarbide single-crystal substrate. When the concentration of the n typeimpurity contained in first region 11 is set too high, punch-through(reach-through) tends to occur. By setting a high concentration of the ntype impurity contained in third region 13 while keeping a lowconcentration of the n type impurity contained in first region 11, thespread of a current in the vicinity of a channel outlet can be ensuredwhile the punch-through is suppressed. Thus, an on resistance of siliconcarbide semiconductor device 200 can be reduced.

Base region 20 is in contact with first impurity region 10. Base region20 is located between first main surface 1 and first impurity region 10.Base region 20 is provided on first impurity region 10. Base region 20has p type (second conductivity type) different from the n type. Baseregion 20 contains a p type impurity (acceptor) for providing p typesuch as Al (aluminum) or B (boron). A concentration of the p typeimpurity contained in base region 20 is not less than 1×10¹⁷ cm⁻³ andnot more than 2×10¹⁸ cm³, for example. Base region 20 has a thickness ofnot less than 0.2 and not more than 0.5 μm, for example.

Source region 30 is provided on base region 20 so as to be separatedfrom first impurity region 10 by base region 20. Source region 30 islocated between first main surface 1 and base region 20. Source region30 contains an n type impurity for providing n type such as phosphorus,and has n type. A concentration of the n type impurity contained insource region 30 is higher than the concentration of the n type impuritycontained in second region 12. The concentration of the n type impuritysuch as phosphorus contained in source region 30 is not less than 2×10¹⁸cm⁻³ and not more than 1×10¹⁹ cm⁻³, for example. Source region 30 has athickness of not less than 0.1 μm and not more than 0.2 μm, for example.

Contact region 42 is a p type region containing a p type impurity suchas aluminum or boron. Contact region 42 is in contact with each ofsource region 30 and base region 20. A concentration of the p typeimpurity contained in contact region 42 is higher than the concentrationof the p type impurity contained in base region 20. The concentration ofthe p type impurity such as aluminum contained in contact region 42 isnot less than 2×10¹⁸ cm⁻³ and not more than 1×10¹⁹ cm⁻³, for example.Contact region 42 has a thickness of not less than 0.1 μm and not morethan 1.3 μm, for example.

First main surface 1 of silicon carbide substrate 100 is provided with atrench 5 (gate trench). Trench 5 has a side surface 3 and a bottomsurface 4. Bottom surface 4 is continuous to side surface 3. Sidesurface 3 extends in a direction substantially perpendicular to firstmain surface 1. Bottom surface 4 is substantially in parallel to firstmain surface 1. A boundary between side surface 3 and bottom surface 4may be formed to have a curvature. Trench 5 has a depth of not less than0.5 μm and not more than 1.5 μm, for example. Trench 5 has a width ofnot less than 0.6 μm and not more than 2.5 μm, for example.

Fourth impurity region 40 is provided between second main surface 2 andbottom surface 4 of trench 5. Fourth impurity region 40 may be incontact with bottom surface 4, as shown in FIG. 1. Fourth impurityregion 40 contains a p type impurity such as aluminum or boron, and hasp type (second conductivity type).

Fourth impurity region 40 is higher in impurity concentration than baseregion 20. Fourth impurity region 40 may be electrically connected tosource electrode 8. A concentration of the p type impurity such asaluminum contained in fourth impurity region 40 is not less than 1×10¹⁸cm⁻³ and not more than 9×10¹⁸ cm⁻³, for example.

As shown in FIG. 1, side surface 3 of trench 5 is in contact with thirdregion 13, second impurity region 20, third impurity region 30, andfourth impurity region 40. First region 11 is spaced apart from sidesurface 3. Second region 12 is spaced apart from side surface 3. Thirdregion 13 is in contact with each of first region 11 and second impurityregion 20. Third region 13 may be in contact with a boundary betweenfirst region 11 and second impurity region 20. Third region 13 is incontact with side surface 3, and protrudes in a direction away from sidesurface 3. First region 11 is in contact with second region 12. Secondregion 12 is electrically connected to third region 13, but may bephysically spaced apart from third region 13.

Bottom surface 4 of trench 5 is in contact with fourth impurity region40, for example. Fourth impurity region 40 is provided to face theentire bottom surface 4, for example. As shown in FIG. 2, in a directionparallel to first main surface 1, fourth impurity region 40 may begreater in width than bottom surface 4. In the direction parallel tofirst main surface 1, a distance from side surface 3 to an end of thirdregion 13 (first distance 111) is shorter than a distance from sidesurface 3 to an end of fourth impurity region 40 (second distance 112).Stated from another perspective, in the direction parallel to first mainsurface 1, an amount of spread of fourth impurity region 40 from sidesurface 3 is larger than an amount of spread of third region 13 fromside surface 3.

The end of third region 13 refers to a position that has a concentrationof 1/e of a maximum value of the concentration of the n type impurity inthird region 13, and that is farthest from side surface 3 in thedirection parallel to first main surface 1. It should be noted that “e”represents a Napier's constant. The end of fourth impurity region 40refers to a position where a carrier concentration exhibits a minimalvalue due to cancellation of the n type impurity in adjacent firstimpurity region 10 and the p type impurity in fourth impurity region 40,and that is farthest from side surface 3 in the direction parallel tofirst main surface 1.

As shown in FIG. 2, as seen in a direction perpendicular to second mainsurface 2, trench 5 may have a substantially rectangular shape. Trench 5extends in a first direction 101 and a second direction 102. Firstdirection 101 is a short direction of trench 5. Second direction 102 isa longitudinal direction of trench 5. First direction 101 is a <1-100>direction, for example. Second direction 102 is a <11-20> direction, forexample. First direction 101 may be a direction defined by projectingthe <1-100> direction on first main surface 1, for example. Seconddirection 102 may be a direction defined by projecting the <11-20>direction on first main surface 1, for example.

As shown in FIG. 2, each of third region 13 and fourth impurity region40 extends in a direction in which trench 5 extends. A longitudinaldirection of third region 13 is the same as the longitudinal directionof trench 5. A short direction of third region 13 is the same as theshort direction of trench 5. A longitudinal direction of fourth impurityregion 40 is the same as the longitudinal direction of trench 5. A shortdirection of fourth impurity region 40 is the same as the shortdirection of trench 5. As shown in FIG. 2, as seen in the directionperpendicular to second main surface 2, fourth impurity region 40 issuperimposed on bottom surface 4 of trench 5. In the short direction oftrench 5, fourth impurity region 40 may be greater in width than trench5. Likewise, in the longitudinal direction of trench 5, fourth impurityregion 40 may be greater in width than trench 5.

Gate insulating film 6 is made of silicon dioxide, for example. Gateinsulating film 6 is provided to be in contact with side surface 3 andbottom surface 4 of trench 5. Gate insulating film 6 is in contact withthird region 13, base region 20, source region 30, and fourth impurityregion 40, at side surface 3 of trench 5. Gate insulating film 6 is incontact with fourth impurity region 40 at bottom surface 4 of trench 5.The configuration is such that a channel region can be formed in baseregion 20 in contact with gate insulating film 6. Gate insulating film 6has a thickness of not less than 40 nm and not more than 150 nm, forexample.

Gate electrode 7 is provided on gate insulating film 6. Gate electrode 7is disposed in contact with gate insulating film 6. The gate electrodeis provided to fill a groove formed by gate insulating film 6. Gateelectrode 7 is made of a conductor such as polysilicon doped with animpurity, for example.

Source electrode 8 is made of a Ni alloy, for example. Source electrode8 is electrically connected to source region 30 on the first mainsurface 1 side of silicon carbide substrate 100. Source electrode 8 isin contact with contact region 42. Source electrode 8 includes an alloylayer in ohmic contact with source region 30. The alloy layer is asilicide with a metal contained in source electrode 8, for example.Source electrode 8 may be made of a material containing Ti, Al and Si.

Interlayer insulating film 23 is provided at a position facing firstmain surface 1 of silicon carbide substrate 100. Specifically,interlayer insulating film 23 is provided in contact with each of gateelectrode 7 and gate insulating film 6 so as to cover gate electrode 7.Interlayer insulating film 23 includes a none-doped silicate glass (NSG)film and a phosphorus silicate glass (PSG) film, for example. The NSGmay be provided on the PSG. Interlayer insulating film 23 electricallyinsulates gate electrode 7 and source electrode 8 from each other.Source wire 21 is provided to cover interlayer insulating film 23 and tobe in contact with source electrode 8. Source wire 21 is electricallyconnected to source region 30 via source electrode 8. Source wire 21 ismade of a material containing AlSiCu, for example. Protective film 22 isprovided on source wire 21 to cover source wire 21. Protective film 22includes a nitride film and polyimide, for example.

Drain electrode 9 is provided in contact with second main surface 2 ofsilicon carbide substrate 100. Drain electrode 9 is electricallyconnected to first impurity region 10 on the second main surface 2 side.Drain electrode 9 is made of a material, such as NiSi (nickel silicide),capable of making ohmic contact with silicon carbide single-crystalsubstrate 15 of n type. Drain electrode 9 is electrically connected tosilicon carbide single-crystal substrate 15.

Operation of MOSFET 200 according to the present embodiment is nowdescribed. When a voltage applied to gate electrode 7 is lower than athreshold voltage, that is, in an off state, a pn junction formedbetween base region 20 and first impurity region 10 is reverse biasedand is non-conducting even if a voltage is applied between sourceelectrode 8 and drain electrode 9. When a voltage equal to or higherthan the threshold voltage is applied to gate electrode 7, on the otherhand, an inversion layer is formed in a channel region near an areawhere base region 20 and gate insulating film 6 are in contact with eachother. As a result, source region 30 and first impurity region 10 areelectrically connected to each other, causing a current to flow betweensource electrode 8 and drain electrode 9. MOSFET 200 operates in amanner described above.

A method of manufacturing silicon carbide semiconductor device 200according to the present embodiment is now described. FIG. 3 is aflowchart schematically showing the method of manufacturing the siliconcarbide semiconductor device according to the present embodiment.

First, a step of preparing a silicon carbide substrate (FIG. 3: S10) isperformed. For example, a silicon carbide single-crystal ingot grownwith a modified Lely method is sliced to cut a substrate, and a surfaceof the substrate is mirror polished, to prepare silicon carbidesingle-crystal substrate 15 (see FIG. 4). Silicon carbide single-crystalsubstrate 15 is made of hexagonal silicon carbide having a polytype of4H, for example. Silicon carbide single-crystal substrate 15 has a thirdmain surface 51, and second main surface 2 opposite to third mainsurface 51. Third main surface 51 has a diameter of 150 mm, for example.Third main surface 51 is a {0001} plane or a plane angled off by notmore than about 8° relative to the {0001} plane, for example. Siliconcarbide single-crystal substrate 15 has a thickness of 400 μm, forexample.

Then, a step of forming a silicon carbide epitaxial layer is performed.For example, carrier gas containing hydrogen, source material gascontaining silane and propane, and dopant gas containing nitrogen aresupplied onto silicon carbide single-crystal substrate 15, and siliconcarbide single-crystal substrate 15 is heated to about 1550° C., forexample, under a pressure of 100 mbar (10 kPa). Silicon carbideepitaxial layer 16 having n type is thus formed on silicon carbidesingle-crystal substrate 15, as shown in FIG. 5. Silicon carbideepitaxial layer 16 has been doped with nitrogen as an n type impurity. Aconcentration of the n type impurity is 8.0×10¹⁵ cm³, for example.Silicon carbide epitaxial layer 16 has a thickness of 10 μm, forexample.

Next, an ion implantation through-mask 60 is formed. Ion implantationthrough-mask 60 is formed by a combination of a thermal oxide film and adeposited oxide film, for example. Ion implantation through-mask 60 hasa thickness of about 50 nm, for example. Then, ions are implanted intosilicon carbide epitaxial layer 16. N (nitrogen) ions, for example, areimplanted through ion implantation through-mask 60 into silicon carbideepitaxial layer 16 in a direction of arrows (direction perpendicular tothird main surface 51). First region 11 having n type is thus formed. Aconcentration of the n type impurity contained in first region 11 is notless than 3×10¹⁶ cm⁻³ and not more than 3×10¹⁷ cm⁻³, for example. Firstregion 11 has a thickness of not less than 0.2 μm and not more than 0.5μm, for example.

Then, Al (aluminum) ions, for example, are implanted through ionimplantation through-mask 60 into a part of first region 11 of siliconcarbide epitaxial layer 16 in the direction of arrows. Second impurityregion 20 having p type is thus formed. A concentration of the p typeimpurity contained in second impurity region 20 is not less than 1×10¹⁷cm⁻³ and not more than 2×10¹⁸ cm⁻³, for example. Second impurity region20 has a thickness of not less than 0.2 μm and not more than 0.5 μm, forexample.

Then, P (phosphorus) ions, for example, are implanted through ionimplantation through-mask 60 into a part of second impurity region 20 ofsilicon carbide epitaxial layer 16 in the direction of arrows. Thirdimpurity region 30 having n type is thus formed (see FIG. 7). Aconcentration of the n type impurity contained in third impurity region30 is not less than 2×10¹⁸ cm⁻³ and not more than 1×10¹⁹ cm³, forexample. Third impurity region 30 has a thickness of not less than 0.1μm and not more than 0.2 μm, for example.

Then, an oxide film mask 63 is formed. Oxide film mask 63 has a firstportion 61 and a second portion 62. Second portion 62 is greater inthickness than first portion 61. Al (aluminum) ions, for example, areimplanted through oxide film mask 63 into a part of second impurityregion 20 and a part of third impurity region 30 of silicon carbideepitaxial layer 16. Contact region 42 having p type is thus formed (seeFIG. 8). A concentration of the p type impurity contained in contactregion 42 is not less than 2×10¹⁸ cm³ and not more than 1×10¹⁹ cm⁻³, forexample. Contact region 42 has a thickness of not less than 0.1 μm andnot more than 0.3 μm, for example. Contact region 42 extends throughthird impurity region 30 and reaches second impurity region 20.

Silicon carbide substrate 100 having first main surface 1 and secondmain surface 2 opposite to first main surface 1 is prepared as set forthabove. Silicon carbide substrate 100 includes first impurity region 10having the first conductivity type, second impurity region 20 providedon first impurity region 10 and having the second conductivity typedifferent from the first conductivity type, and third impurity region 30provided on second impurity region 20 so as to be separated from firstimpurity region 10, being in contact with second main surface 2, andhaving the first conductivity type. First impurity region 10 has firstregion 11 in contact with second impurity region 20, and second region12 located opposite to second impurity region 20 with respect to firstregion 11.

Next, a step of forming a mask on a first main surface (S20: FIG. 3) isperformed. Specifically, a mask 64 is formed on first main surface 1.Mask 64 functions as an etching mask. Mask 64 is made of a materialincluding a deposited oxide film, for example. Mask 64 has a thicknessof 1.6 μm, for example. Then, mask 64 over a region where a first trench70 is to be formed is RF etched using CHF₃ and O₂, to form an opening inmask 64.

Next, a step of forming a first trench (S30: FIG. 3) is performed.Silicon carbide substrate 100 is etched using mask 64 having the openingformed over the region where first trench 70 is to be formed. Forexample, using mask 64, third impurity region 30 and second impurityregion 20 are anisotropically etched under an atmosphere of SF₆ and O₂.The anisotropic etching refers to electron cyclotron resonance (ECR)plasma etching, for example. First trench 70 is thus formed in firstmain surface 1 of silicon carbide substrate 100.

As shown in FIG. 9, first trench 70 has a first side surface 71 and afirst bottom surface 72. First side surface 71 is in contact with secondimpurity region 20 and third impurity region 30. First bottom surface 72is in contact with second impurity region 20. First trench 70 has adepth of not less than 0.3 μm and not more than 0.8 μm, for example.First trench 70 has a width of not less than 0.5 μm and not more than 2μm, for example.

Next, a step of forming a fourth impurity region by ion implantation(S40: FIG. 3) is performed. Specifically, ions are implanted into firsttrench 70 using mask 64. In this step, mask 64 functions as an ionimplantation mask. Specifically, Al (aluminum) ions are implanted into apart of first region 11 and a part of second region 12, with mask 64being provided on first main surface 1. Ion implantation energy is notless than 700 keV, for example. Fourth impurity region 40 having p typeis thus formed.

Fourth impurity region 40 is in contact with first impurity region 10.Specifically, fourth impurity region 40 is in contact with each of firstregion 11 and second region 12. In the direction parallel to first mainsurface 1, fourth impurity region 40 is greater in width than firstbottom surface 72 of first trench 70. Fourth impurity region 40 spreadsabout 0.2 μm from each of paired first side surfaces 71 of first trench70. A concentration of the p type impurity contained in fourth impurityregion 40 is not less than 1×10¹⁸ cm⁻³ and not more than 9×10¹⁸ cm⁻³,for example. Fourth impurity region 40 has a thickness of not less than0.5 μm and not more than 1.5 μm, for example.

Next, a step of forming a third region by ion implantation (S50: FIG. 3)is performed. Specifically, N (nitrogen) ions are implanted into a partof first region 11, a part of second impurity region 20, and a part offourth impurity region 40, with mask 64 being provided on first mainsurface 1. Ion implantation energy is not less than 400 keV, forexample. Third region 13 having n type is thus formed. The ionimplantation energy in the step of forming a third region by ionimplantation (S50: FIG. 3) is smaller than the ion implantation energyin the step of forming a fourth impurity region by ion implantation(S40: FIG. 3). Third region 13 is formed on the first main surface 1side with respect to fourth impurity region 40.

As shown in FIG. 10, third region 13 is in contact with first impurityregion 10. Specifically, third region 13 is in contact with each offirst region 11, second impurity region 20 and fourth impurity region40. In the direction parallel to first main surface 1, third region 13is greater in width than first bottom surface 72 of first trench 70, andis smaller in width than fourth impurity region 40. Third region 13spreads about 0.1 μm from each of paired first side surfaces 71 of firsttrench 70. Third region 13 is higher in impurity concentration thanfirst region 11. A concentration of the n type impurity contained inthird region 13 is not less than 2×10¹⁶ cm⁻³ and not more than 5×10¹⁷cm⁻³, for example. Third region 13 has a thickness of not less than 0.3μm and not more than 1 for example. Third region 13 may be formed to bespaced apart from first bottom surface 72 of first trench 70. In thiscase, second impurity region 20 is provided between third region 13 andfirst bottom surface 72.

When using silicon carbide substrate 100 having first main surface 1inclined relative to a {0001} plane (off substrate), channeling can besuppressed. Thus, ion implantation can be effectively performed in adirection perpendicular to first main surface 1. When ion implantationis performed in a direction inclined relative to the normal of firstmain surface 1, lateral spread can be facilitated.

Next, a step of forming a second trench by extending the first trench(S60: FIG. 3) is performed. Specifically, using mask 64, third region13, second impurity region 20 and fourth impurity region 40 are ECRplasma etched under an atmosphere of SF₆ and O₂. First trench 70 is thusextended to form a second trench 5. First trench 70 is extended mainlyin a depth direction.

As shown in FIG. 11, second trench 5 has a second side surface 3 and asecond bottom surface 4. Second side surface 3 is in contact with thirdregion 13, second impurity region 20, third impurity region 30, andfourth impurity region 40. Second bottom surface 4 is in contact withfourth impurity region 40. Second trench 5 is greater in depth thanfirst trench 70. Second trench 5 has a depth of not less than 0.5 μm andnot more than 1.5 μm, for example. Second trench 5 is substantiallyequal in width to first trench 70. Second trench 5 has a width of notless than 0.6 μm and not more than 2.5 μm, for example. Second trench 5may be greater in width than first trench 70.

Next, a step of annealing the silicon carbide substrate (S70: FIG. 3) isperformed. First, mask 64 is removed from first main surface 1 ofsilicon carbide substrate 100 (see FIG. 12). Then, silicon carbidesubstrate 100 is disposed in a case 80 made of silicon carbide (see FIG.13). Case 80 has a containing portion 81 and a cover portion 82. Siliconcarbide substrate 100 is disposed in containing portion 81. Coverportion 82 is disposed on containing portion 81. Silicon carbidesubstrate 100 is thus enclosed within case 80.

Silicon carbide substrate 100 is heated while being enclosed within case80. Since silicon carbide substrate 100 is disposed in case 80 made ofsilicon carbide, silicon carbide substrate 100 is heated in a siliconcarbide atmosphere. Silicon carbide is thus maintained in a thermalequilibrium state. In the silicon carbide atmosphere, silicon carbidesubstrate 100 is annealed at not less than 1400° C. and not more than1900° C. Preferably, silicon carbide substrate 100 is annealed at notless than 1500° C. and not more than 1800° C. An annealing temperaturefor silicon carbide substrate 100 is 1700° C., for example. An annealingtime is 10 minutes, for example.

Thermal etching of second trench 5 and activation of the ion-implantedimpurities are simultaneously performed as set forth above. The thermaletching removes a plasma damaged layer at each of second side surface 3and second bottom surface 4 of second trench 5, while rounding eachcorner formed by second side surface 3 and second bottom surface 4. As aresult of the rounded corner, concentration of electric field at aboundary portion between second side surface 3 and second bottom surface4 can be suppressed.

Next, a step of forming a gate insulating film (S80: FIG. 3) isperformed. Specifically, gate insulating film 6 in contact with firstmain surface 1, second side surface 3 and second bottom surface 4 isformed (see FIG. 14). Gate insulating film 6 is a deposited oxide film,for example. Gate insulating film 6 is in contact with third region 13,fourth impurity region 40, base region 20, and source region 30, atsecond side surface 3. Gate insulating film 6 is in contact with fourthimpurity region 40 at second bottom surface 4. Gate insulating film 6 isin contact with source region 30 at first main surface 1. Gateinsulating film 6 has a thickness of not less than 40 nm and not morethan 150 nm, for example.

Next, a NO annealing step is performed. Specifically, silicon carbidesubstrate 100 having gate insulating film 6 formed on first main surface1 is subjected to heat treatment at a temperature of not less than 1100°C. and not more than 1300° C., for example, in an atmosphere containingnitrogen. Examples of the gas containing nitrogen include nitrogenmonoxide diluted with nitrogen by 10%. Silicon carbide substrate 100 isannealed for not less than 30 minutes and not more than 360 minutes, forexample, in the gas containing nitrogen.

Next, a step of forming a gate electrode (S90: FIG. 3) is performed.Specifically, gate electrode 7 is formed on gate insulating film 6 tofill a groove formed by gate insulating film 6. Gate electrode 7 is madeof a material including polysilicon containing an impurity, for example.Then, interlayer insulating film 23 is formed to cover gate electrode 7.Interlayer insulating film 23 includes a NSG film and a PSG film, forexample.

Next, a step of forming a source electrode (S100: FIG. 3) is performed.Specifically, interlayer insulating film 23 and gate insulating film 6are removed from a region where source electrode 8 is to be formed, toexpose each of source region 30 and contact region 42 at interlayerinsulating film 23. Then, source electrode 8 is formed by sputtering,for example, to be in contact with both source region 30 and contactregion 42 at first main surface 1. Source electrode 8 contains a Nialloy, for example. Source electrode 8 may be made of a materialcontaining TiAlSi. Then, silicon carbide substrate 100 having sourceelectrode 8 is subjected to rapid thermal anneal (RTA) of not less than900° C. and not more than 1100° C., for example, for about 2 minutes. Atleast a part of source electrode 8 thus reacts with silicon contained insilicon carbide substrate 100 and is silicided. Source electrode 8 inohmic contact with source region 30 is thus formed. Preferably, sourceelectrode 8 is in ohmic contact with each of source region 30 andcontact region 42.

Next, source wire 21 is formed to be in contact with source electrode 8and to cover interlayer insulating film 23. Source wire 21 is preferablymade of a material containing Al, for example, a material containingAlSiCu. Then, protective film 22 is formed to cover source wire 21.Protective film 22 is made of a material including a nitride film andpolyimide, for example.

Next, a step of forming a drain electrode (S110: FIG. 3) is performed.Specifically, drain electrode 9 made of NiSi, for example, is formed incontact with second main surface 2 of silicon carbide substrate 100.Drain electrode 9 may be made of TiAlSi, for example. Drain electrode 9is preferably formed by sputtering, but may be formed by vapordeposition. After being formed, drain electrode 9 is heated by laserannealing, for example. At least a part of drain electrode 9 is thussilicided and in ohmic contact with silicon carbide single-crystalsubstrate 15. MOSFET 200 shown in FIG. 1 is manufactured in a mannerdescribed above.

A method of manufacturing silicon carbide semiconductor device 200according to a variation of the present embodiment is now described.FIG. 15 is a flowchart schematically showing the method of manufacturingsilicon carbide semiconductor device 200 according to the variation ofthe present embodiment.

First, as shown in FIGS. 3 to 9, the step of preparing a silicon carbidesubstrate (S10: FIG. 15), the step of forming a mask on a first mainsurface (S20: FIG. 15), and the step of forming a first trench (S30:FIG. 15) are performed.

Next, the step of forming a third region by ion implantation (S50: FIG.15) is performed. Specifically, N (nitrogen) ions are implanted into apart of first region 11 and a part of second impurity region 20, withmask 64 being provided on first main surface 1 (see FIG. 16). As shownin FIG. 16, N (nitrogen) ions may be implanted into a part of secondregion 12. Ion implantation energy is not less than 400 keV, forexample. Third region 13 having n type is thus formed.

Third region 13 is in contact with each of first region 11, secondregion 12 and second impurity region 20, for example. In the directionparallel to first main surface 1, third region 13 is greater in widththan first bottom surface 72 of first trench 70. Third region 13 spreadsabout 0.1 μm from each of paired first side surfaces 71 of first trench70. A concentration of the n type impurity contained in third region 13is not less than 2×10¹⁶ cm⁻³ and not more than 5×10¹⁷ cm⁻³, for example.Third region 13 has a thickness of not less than 0.3 μm and not morethan 1 μm, for example. Third region 13 may be formed to be spaced apartfrom first bottom surface 72 of first trench 70. In this case, secondimpurity region 20 is provided between third region 13 and first bottomsurface 72.

Next, the step of forming a second trench by extending the first trench(S60: FIG. 15) is performed. Specifically, using mask 64, third region13 and second impurity region 20 are ECR plasma etched under anatmosphere of SF₆ and O₂. First trench 70 is thus extended to formsecond trench 5.

As shown in FIG. 17, second trench 5 has second side surface 3 andsecond bottom surface 4. Second side surface 3 is in contact with thirdregion 13, second impurity region 20, and third impurity region 30.Second bottom surface 4 is in contact with third region 13. Secondtrench 5 is greater in depth than first trench 70. Second trench 5 has adepth of not less than 0.5 μm and not more than 1.5 μm, for example.Second trench 5 is substantially equal in width to first trench 70.Second trench 5 has a width of not less than 0.6 μm and not more than2.5 μm, for example. Second trench 5 may be greater in width than firsttrench 70.

Next, a step of forming a fourth impurity region by ion implantation(S65: FIG. 15) is performed. Specifically, ions are implanted into firsttrench 70 using mask 64. In this step, mask 64 functions as an ionimplantation mask. Specifically, Al (aluminum) ions are implanted into apart of third region 13 and a part of second region 12, with mask 64being provided on first main surface 1. Ion implantation energy is notless than 700 keV, for example. Fourth impurity region 40 having p typeis thus formed.

Fourth impurity region 40 is in contact with each of first region 11,second region 12, and third region 13. In the direction parallel tofirst main surface 1, fourth impurity region 40 is greater in width thansecond bottom surface 4 of second trench 5. Fourth impurity region 40spreads about 0.2 μm from each of paired second side surfaces 3 ofsecond trench 5. A concentration of the p type impurity contained infourth impurity region 40 is not less than 1×10¹⁸ cm⁻³ and not more than9×10¹⁸ cm³, for example. Fourth impurity region 40 has a thickness ofnot less than 0.5 μm and not more than 1.5 μm, for example.

Next, as shown in FIGS. 12 to 14, the step of annealing the siliconcarbide substrate (S70: FIG. 15) and the step of forming a gateinsulating film (S80: FIG. 15) are performed. Next, the step of forminga gate electrode (S90: FIG. 15), the step of forming a gate electrode(S100: FIG. 15), and the step of forming a drain electrode (S110: FIG.15) are performed. MOSFET 200 shown in FIG. 1 is manufactured in amanner described above.

Second Embodiment

The configuration of silicon carbide semiconductor device 200 accordingto a second embodiment is now described. Silicon carbide semiconductordevice 200 according to the second embodiment is primarily different inconfiguration from silicon carbide semiconductor device 200 according tothe first embodiment in that fourth impurity region 40 is spaced apartfrom bottom surface 4 of trench 5, and is otherwise similar inconfiguration to silicon carbide semiconductor device 200 according tothe first embodiment. The configuration different from silicon carbidesemiconductor device 200 according to the first embodiment is mainlydescribed below.

As shown in FIG. 18, in silicon carbide semiconductor device 200according to the second embodiment, fourth impurity region 40 is spacedapart from bottom surface 4 of trench 5. Third region 13 is providedbetween bottom surface 4 and fourth impurity region 40. Third region 13is in contact with each of bottom surface 4 and fourth impurity region40. Side surface 3 of trench 5 is in contact with third region 13,second impurity region 20, and third impurity region 30. First region 11may be in contact with side surface 3. Second region 12 is spaced apartfrom side surface 3. Third region 13 is in contact with each of firstregion 11 and second region 12.

Bottom surface 4 of trench 5 is in contact with third region 13. Thirdregion 13 is provided to face the entire bottom surface 4, for example.In the direction parallel to first main surface 1, third region 13 isgreater in width than bottom surface 4. In the direction parallel tofirst main surface 1, the distance from side surface 3 to the end ofthird region 13 (first distance) is shorter than the distance from sidesurface 3 to the end of fourth impurity region 40 (second distance).Stated from another perspective, in the direction parallel to first mainsurface 1, fourth impurity region 40 is greater in width than thirdregion 13.

As shown in FIG. 18, in the direction perpendicular to second mainsurface 2, third region 13 at the center of bottom surface 4 may besmaller in thickness than third region 13 at the boundary between bottomsurface 4 and side surface 3. In the direction perpendicular to secondmain surface 2, fourth impurity region 40 at the center of bottomsurface 4 may be greater in thickness than fourth impurity region 40 atthe boundary between bottom surface 4 and side surface 3.

Although silicon carbide semiconductor device 200 according to thepresent disclosure has been described above with reference to a MOSFEThaving trench 5 as an example, silicon carbide semiconductor device 200according to the present disclosure is not limited thereto. Siliconcarbide semiconductor device 200 according to the present disclosure maybe, for example, an insulated gate bipolar transistor (IGBT). Although ntype and p type have been described above as the first conductivity typeand the second conductivity type, respectively, p type may be the firstconductivity type and n type may be the second conductivity type. Theconcentration of a p type impurity and the concentration of an n typeimpurity in each impurity region described above can be measured, forexample, with a scanning capacitance microscope (SCM) or secondary ionmass spectrometry (SIMS).

Functions and effects of silicon carbide semiconductor device 200 andthe method of manufacturing the same according to the above embodimentwill now be described.

According to silicon carbide semiconductor device 200 according to theabove embodiment, fourth impurity region 40 having p type is providedbetween second main surface 2 and bottom surface 4 of trench 5. Thus,breakdown of gate insulating film 6 due to application of high electricfield to gate insulating film 6 in the vicinity of bottom surface 4 canbe suppressed. As a result, a high breakdown voltage of silicon carbidesemiconductor device 200 can be maintained. First impurity region 10 hasfirst region 11 being in contact with second impurity region 20 andhaving the first impurity concentration, second region 12 providedbetween first region 11 and second main surface 2 and having the secondimpurity concentration lower than the first impurity concentration, andthird region 13 being continuous to first region 11 and having the thirdimpurity concentration higher than the first impurity concentration. Byproviding third region 13 higher in impurity concentration than firstregion 11, narrowing of a current in the vicinity of the channel outletcan be suppressed. Thus, the on resistance of silicon carbidesemiconductor device 200 can be reduced.

According to silicon carbide semiconductor device 200 according to theabove embodiment, fourth impurity region 40 may be in contact withbottom surface 4. Thus, application of high electric field to gateinsulating film 6 in the vicinity of bottom surface 4 can be furthersuppressed. As a result, a higher breakdown voltage of silicon carbidesemiconductor device 200 can be maintained.

Further, according to silicon carbide semiconductor device 200 accordingto the above embodiment, in the direction parallel to first main surface1, the distance from side surface 3 to the end of third region 13 may beshorter than the distance from side surface 3 to the end of fourthimpurity region 40. In this case, when third region 13 is seen fromsecond region 12, third region 13 is blocked by fourth impurity region40. Thus, application of high electric field to gate insulating film 6in the vicinity of third region 13 can be suppressed. As a result, ahigh breakdown voltage of silicon carbide semiconductor device 200 canbe maintained.

According to the method of manufacturing silicon carbide semiconductordevice 200 according to the present disclosure, third impurity region 30and second impurity region 20 are etched using mask 64, to form firsttrench 70 in first main surface 1. The ions are implanted into firsttrench 70 using mask 64, to form third region 13 being in contact withfirst impurity region 10 and having the first conductivity type. Thirdregion 13 and second impurity region 20 are etched using mask 64, toextend first trench 70 to form second trench 5. The ions are implantedinto second trench 5 using mask 64, to form fourth impurity region 40being in contact with first impurity region 10 and having the secondconductivity type. By performing the etching and ion implantation(self-alignment) using the same mask 64 as described above, theoccurrence of misalignment can be suppressed. Thus, third region 13 andfourth impurity region 40 can be formed with a high degree of accuracy.The number of steps can also be reduced to cut the process cost.

According to the method of manufacturing silicon carbide semiconductordevice 200 according to the present disclosure, third impurity region 30and second impurity region 20 are etched using mask 64, to form firsttrench 70 in first main surface 1. The ions are implanted into firsttrench 70 using mask 64, to form third region 13 being in contact withfirst impurity region 10 and having the first conductivity type. Thirdregion 13 and second impurity region 20 are etched using mask 64, toextend first trench 70 to form second trench 5. The ions are implantedinto second trench 5 using mask 64, to form fourth impurity region 40being in contact with first impurity region 10 and having the secondconductivity type. By performing the etching and ion implantation(self-alignment) using the same mask 64 as described above, theoccurrence of misalignment can be suppressed. Thus, third region 13 andfourth impurity region 40 can be formed with a high degree of accuracy.The number of steps can also be reduced to cut the process cost.

According to the method of manufacturing silicon carbide semiconductordevice 200 according to the present disclosure, after forming secondtrench 5, silicon carbide substrate 100 may be annealed at not less than1400° C. and not more than 1900° C. in the silicon carbide atmosphere.Thus, thermal etching of second trench 5 and activation of theion-implanted impurities are simultaneously performed. Accordingly, ascompared to an example where thermal etching of second trench 5 andactivation of the ion-implanted impurities are performed in differentsteps, the number of steps can be reduced. As a result, the process costcan be cut further.

EXAMPLE

A characteristic on resistance and a breakdown voltage of a MOSFET werecalculated through simulation while first distance 111 (the distancefrom side surface 3 of trench 5 to the end of third region 13 in adirection parallel to second main surface 2) in the MOSFET according tothe first embodiment (see FIG. 1) was varied. Results of the calculationare described. The characteristic on resistance and the breakdownvoltage of the MOSFET were calculated while a value of first distance111 was varied from 0 μm to 0.7 μm. An n type impurity concentration infirst region 11 was set to 1×10¹⁷ cm⁻³. An n type impurity concentrationin second region 12 was set to 8×10¹⁵ cm⁻³. An n type impurityconcentration in third region 13 was set to 2×10¹⁷ cm⁻³. A p typeimpurity concentration in fourth impurity region 40 was set to 3×10¹⁸cm⁻³.

Relation between the distance from side surface 3 of trench 5 (firstdistance 111) and the characteristic on resistance, and relation betweenthe distance from side surface 3 of trench 5 (first distance 111) andthe breakdown voltage are described with reference to FIG. 19. In thefigure, the horizontal axis represents the distance (μm) from sidesurface 3 of trench 5, the left vertical axis represents thecharacteristic on resistance (mΩcm²) of the MOSFET, and the rightvertical axis represents the breakdown voltage (V) of the MOSFET. In thefigure, a white circle represents a value of the breakdown voltage, anda white triangle represents a value of the characteristic on resistance.A breakdown voltage specification was 1200 V.

When the distance from side surface 3 of trench 5 is in the range fromnot less than 0 μm to less than 0.4 μm, the characteristic on resistancedecreases gradually. When the distance from side surface 3 of trench 5reaches 0.4 μm or more, however, the characteristic on resistance ismaintained at a substantially constant value. As for the breakdownvoltage, the specification is satisfied over the entire range. When thedistance from side surface 3 of trench 5 is in the range from not lessthan 0 μm to not more than 0.3 μm, the breakdown voltage exhibits aparticularly high value. When the distance from side surface 3 of trench5 exceeds 0.3 μm, however, the breakdown voltage decreases gradually.From the results above, it was confirmed that a MOSFET including thirdregion 13 can reduce the characteristic on resistance while maintaininga high breakdown voltage, as compared to a MOSFET not including thirdregion 13.

It should be understood that the embodiments and example disclosedherein are illustrative and non-restrictive in every respect. The scopeof the present invention is defined by the terms of the claims ratherthan the description above, and is intended to include any modificationswithin the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1 first main surface; 2 second main surface; 3 side surface (second sidesurface); 4 bottom surface (second bottom surface); 5 second trench(trench); 6 gate insulating film; 7 gate electrode; 8 source electrode;9 drain electrode; 10 first impurity region; 11 first region; 12 secondregion; 13 third region; 15 silicon carbide single-crystal substrate; 16silicon carbide epitaxial layer; 20 base region (second impurityregion); 21 source wire; 22 protective film; 23 interlayer insulatingfilm; 30 source region (third impurity region); 40 fourth impurityregion; 42 contact region; 51 third main surface; 60 ion implantationthrough-mask; 61 first portion; 62 second portion; 63 oxide film mask;64 mask; 70 first trench; 71 first side surface; 72 first bottomsurface; 80 case; 81 containing portion; 82 cover portion; 100 siliconcarbide substrate; 101 first direction; 102 second direction; 111 firstdistance; 112 second distance; 200 silicon carbide semiconductor device(MOSFET).

The invention claimed is:
 1. A silicon carbide semiconductor devicecomprising a silicon carbide substrate having a first main surface and asecond main surface opposite to the first main surface, the first mainsurface being provided with a trench having a side surface and a bottomsurface continuous to the side surface, the silicon carbide substrateincluding a first impurity region located between the first main surfaceand the second main surface and having a first conductivity type, asecond impurity region located between the first main surface and thefirst impurity region, provided on the first impurity region, and havinga second conductivity type different from the first conductivity type, athird impurity region located between the first main surface and thesecond impurity region, provided on the second impurity region so as tobe separated from the first impurity region, and having the firstconductivity type, and a fourth impurity region provided between thesecond main surface and the bottom surface and having the secondconductivity type, the first impurity region having a first region beingin contact with the second impurity region and having a first impurityconcentration, a second region being continuous to the first region,provided between the first region and the second main surface, andhaving a second impurity concentration lower than the first impurityconcentration, and a third region being continuous to the first regionand having a third impurity concentration higher than the first impurityconcentration, the side surface being in contact with the third region,the second impurity region, and the third impurity region, the fourthimpurity region is in contact with the bottom surface, and in adirection parallel to the first main surface, a distance from the sidesurface to an end of the third region is shorter than a distance fromthe side surface to an end of the fourth impurity region.
 2. A siliconcarbide semiconductor device comprising a silicon carbide substratehaving a first main surface and a second main surface opposite to thefirst main surface, the first main surface being provided with a trenchhaving a side surface and a bottom surface continuous to the sidesurface, the silicon carbide substrate including a first impurity regionlocated between the first main surface and the second main surface andhaving a first conductivity type, a second impurity region locatedbetween the first main surface and the first impurity region, providedon the first impurity region, and having a second conductivity typedifferent from the first conductivity type, a third impurity regionlocated between the first main surface and the second impurity region,provided on the second impurity region so as to be separated from thefirst impurity region, and having the first conductivity type, and afourth impurity region provided between the second main surface and thebottom surface and having the second conductivity type, the firstimpurity region having a first region being in contact with the secondimpurity region and having a first impurity concentration, a secondregion being continuous to the first region, provided between the firstregion and the second main surface, and having a second impurityconcentration lower than the first impurity concentration, and a thirdregion being continuous to the first region and having a third impurityconcentration higher than the first impurity concentration, the sidesurface being in contact with the third region, the second impurityregion, and the third impurity region, the fourth impurity region isspaced apart from the bottom surface, and the third region is providedbetween the bottom surface and the fourth impurity region, and is incontact with each of the bottom surface and the fourth impurity region.3. The silicon carbide semiconductor device according to claim 2,wherein the third region is in contact with the second region, and in adirection parallel to the first main surface, a distance from the sidesurface to an end of the third region is shorter than a distance fromthe side surface to an end of the fourth impurity region.
 4. A method ofmanufacturing a silicon carbide semiconductor device, the methodcomprising: preparing a silicon carbide substrate having a first mainsurface and a second main surface opposite to the first main surface,the silicon carbide substrate including a first impurity region locatedbetween the first main surface and the second main surface and having afirst conductivity type, a second impurity region located between thefirst main surface and the first impurity region, provided on the firstimpurity region, and having a second conductivity type different fromthe first conductivity type, and a third impurity region located betweenthe first main surface and the second impurity region, provided on thesecond impurity region so as to be separated from the first impurityregion, being in contact with the first main surface, and having thefirst conductivity type, the first impurity region having a first regionin contact with the second impurity region, and a second region beingcontinuous to the first region and located opposite to the secondimpurity region with respect to the first region; forming a mask on thefirst main surface; etching the third impurity region and the secondimpurity region using the mask, to form a first trench in the first mainsurface; implanting ions into the first trench using the mask, to form afourth impurity region being in contact with the first impurity regionand having the second conductivity type; implanting ions into the firsttrench using the mask, to form a third region being in contact with thefirst impurity region and having the first conductivity type; andetching the third region, the second impurity region and the fourthimpurity region using the mask, to extend the first trench to form asecond trench, a side surface of the second trench being in contact withthe third region, the second impurity region, and the third impurityregion.
 5. The method of manufacturing a silicon carbide semiconductordevice according to claim 4, further comprising, after forming thesecond trench, annealing the silicon carbide substrate at not less than1400° C. and not more than 1900° C. in a silicon carbide atmosphere. 6.A method of manufacturing a silicon carbide semiconductor device, themethod comprising: preparing a silicon carbide substrate having a firstmain surface and a second main surface opposite to the first mainsurface, the silicon carbide substrate including a first impurity regionlocated between the first main surface and the second main surface andhaving a first conductivity type, a second impurity region locatedbetween the first main surface and the first impurity region, providedon the first impurity region, and having a second conductivity typedifferent from the first conductivity type, and a third impurity regionlocated between the first main surface and the second impurity region,provided on the second impurity region so as to be separated from thefirst impurity region, being in contact with the first main surface, andhaving the first conductivity type, the first impurity region having afirst region in contact with the second impurity region, and a secondregion being continuous to the first region and located opposite to thesecond impurity region with respect to the first region; forming a maskon the first main surface; etching the third impurity region and thesecond impurity region using the mask, to form a first trench in thefirst main surface; implanting ions into the first trench using themask, to form a third region being in contact with the first impurityregion and having the first conductivity type; etching the third regionand the second impurity region using the mask, to extend the firsttrench to form a second trench; and implanting ions into the secondtrench using the mask, to form a fourth impurity region being in contactwith the first impurity region and having the second conductivity type,a side surface of the second trench being in contact with the thirdregion, the second impurity region, and the third impurity region.